还应该满足(x1-x2)²=(x3-x4)²
即(x1+x2)²-4x1x2=(x3+x4)²-4x3x4
即p²-4q=q²-4p
即(p+q)(p-q)-4(q-p)=0
∴(p-q)(p+q+4)=0
∴p=q或p+q=-4